Electromagnetic wave attenuator



April 25, 1961 R. c. BUNDY ELECTROMAGNETIC WAVE ATTENUATOR Filed Oct. 18, 1957 ROMP w whww a m g M m J j a Z m J M E j 'thin films or coatings on the base material.

United States Patent ELECTROMAGNETIC WAVE ATTENUATOR Robert C. Bundy, Benson, Ariz., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 18, 1957, Ser. No. 691,038 2 Claims. (Cl. 333-81) This invention relates to electromagnetic wave attenuators and more particularly to a low-power dissipativ attenuator of the resistive coating type.

Many devices are available which provide fixed or variable attenuation of the main power flow of electromagnetic waves through an electromagnetic wave conductor. For low power ranges of electromagnetic waves, support elements of dielectric base materials provided with a thin uniform coating of a power absorbingmaterial such as carbon or aquadag have been extensively utilized. Examples of these lossy material coated support elements are IRC resistance loads, metallized glass loads and many others. A summary which includes a number of such devices is found in chapter 12 of the book entitled Technique of Microwave Measurements, by Montgomery, volume 11 of the Radiation Laboratory Series, published by the Massachusetts Institute of Technology (1947). These low-power absorbers may be inserted into a waveguide to provide a fixed or variable attenuation prior to a utilization point or to serve as terminations.

The desirable characteristics of a low-power attenuator include, inter alia, maximum absorption per unit length of absorber and minimum frequency dependence which is the same as maximum bandwidth. In the past, efforts to improve the desirable characteristics of these attenuators have mainly centered around finding more effective lossy materials for coating the support base. Such efforts are therefore limited by available resistive materials.

It is an object of thisinvention 'to provide an improved electromagnetic wave attenuator of the lossy material coated support type having an increased absorption per unit length.

It is another object of this invention to provide an improved electromagnetic wave attenuator of the lossy material coated support type having an increased broad band characteristic.

It is a further object of this invention to provide an improved electromagnetic wave attenuator which is more broad band in operation and has a higher power absorp tion per unit length than attenuators heretofore known.

It is a still further object of this invention to provide an improved electromagnetic wave attenuator of the lossy material coated support type of radically different design.

In accordance with one embodiment of this invention, a support element of a dielectric base material may be provided with discrete and separated ribbons of power absorbing or lossy material. These ribbons are relatively The width of the ribbons and the width of the spaces between the ribbons are arranged 'with respect to the wavelength of the electromagnetic wave to be attenuated. The discontinuity of the resistive coating in accordance with thisinvention has been found to provide greater absorption per unit length and smaller frequency dependence than was obtainable with previous attenuators.

Fig. 1 is a perspective view of a rectangular wave Patented Apr. 25, 1961 2 guide containing an electromagnetic wave attenuator in accordance with this invention;

Figs. 2, 3 and 4, are respectively elevation views with thickness indicated of different embodhrnents of the attenuator of this invention showing dimensional details of the width of the ribbons of the lossy material;

Figs. 5a, 5b, 5c and 5d are broken views in elevation of impedance matching end portions which may be used with the attenuator of this invention; and

Fig. 6 is an illustrative graph in Cartesian coordinates of several curves showing the improved characteristics of the attenuators of Figs. 3 and 4.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the vention.

Referring now to the drawing-and particularly to Fig. 1, there is shown a waveguide 10 for providing a main power path through which electromagnetic waves may be propagated. Positioned inside the waveguide 10 is an electromagnetic wave attenuator 12 in accordance with this invention which comprises a card or element 14 of dielectric base material and a plurality of ribbons 16 of a lossy (such as resistive) material coated upon the card 14. The ribbons '16 are separated from one another by the uncoated interjacent spaces 14-. The ribbons 16 may be affixed upon both sides of the card 14 or upon one side only. In many'applications, the attenuator is movably supported with the waveguide to provide a variable attenuation. In such a case it is preferable to afiix the discontinuous resistive coating 16 only to the side of the card 14 closest to one of the narrow walls of the waveguide 10. In this manner, when the attenuator is moved all the way to that narrow wall, zero absorption is obtainable.

An attenuator 12 in accordance with this invention is shown in greater detail in Fig. 2 and comprises a rectangular support card 14 and a gaping thin coating of power absorbing material forming a plurality of vartically extending ribbons 22 thereon. The support card 14 as such may be substantially identical in shape, material and purpose to the one utilized in the prior art attenuators. The support card 14 is made of a dielectric base material such as glass, ceramic, mica or plastic and dimensioned for insertion into the main power path. Its thickness should be as thin as possible so as to create a minimum disturbance within the path, yet thick enough to provide a rigid mechanical support for the lossy material which remains planar upon rise of the temperature when power is absorbed and dissipatedas heat. The length of the support 14 is of course related to the overall length of the attenuator and thereby to the attenuation itself and is chosen to provide the desired amount of attenuation.

The ribbons, strips or segments 22 of resistive or lossy material represent the same element as the ribbons 16 of Fig. l. The resistive material itself, the method of affixing and the thickness of the coating may be substantially the same as that employed in prior art low-power attenuators of the uniform continuous films type. For example, lossy materials which may be utilized include aquadag, carbon and copper. The thickness of the film or coating of the lossy material deposited upon the support element 14 ranges from a few ten-thousandths 3 to a few thousandths of an inch. Methods of affixing the lossy material include painting, spraying, depositing and others and are well known in the art. The ribbons or successive segments 2-2 of the gaping coating are separated from one another by the interjacent spaces 24 which are uncoated portions of the support element 14.

It has been found that if the attenuator of Fig. 2 is utilized as an absorption element within a waveguide, the attenuation per unit length is greater than that of the conventional prior art attenuator having a uniform and continuous coating. This phenomenon is not completely understood, but is believed to result from some form of resonance effect.

In the absorber of Fig. 3, the support card 14 has aflixed to it a film of power absorbing material forming the ribbons 32 which are separated by the interjacent spaces 34. The elements 32 refer again to the ribbons 16 of Fig. 1 and have been designated by a different reference character to more clearly define a dimensional relationship for optimum absorption per unit length. It has been found that a definite relationship exists between the absorption per unit length and the width of the ribbons, and of the interjacent spaces. As the interjacent space is increased, the absorption per unit length is increased until the width is equal to one-eighth of the wave absorbed. As the width is still further increased, the absorption per unit length decreases. In accordance with these teachings the absorber shown in Fig. 3, provides optimum absorption when the ribbons 32 and the interjacent spaces 34 each are equal to one-eighth of where is the mean wavelength of the electromagnetic waves to be absorbed.

In the absorber of Fig. 4 the support card 14 has affixed to it, as before, a gaping film of power absorbing material forming the three ribbons 42, 46 and 59 which are respectively separated by the interjacent spaces 44, 48 and 52. The elements 42, 46 and 54) again refer to the ribbon 16 of Fig. 1 and have been re-designated by reference characters 42, 46 and St to define a different dimensional relationship for optimum bandwidth. The card 14 and the ribbons 42, 46 and 50, except for the dimensional relationship hereinafter specified are again similar to the absorber described in connection with Fig. 2. For maximum broad band operation it has been found that the combined width of ribbon 42 and interjacent space 44 should be equal to one-quarter of )t, where A, is the smallest wavelength in the band of electromagnetic waves to be absorbed. Further, the combined width of ribbon 50 and interjacent space 52 should be equal to one-quarter of M, where M, is the largest wavelength in the band of electromagnetic waves to be absorbed. The combined width of intermediate ribbons and their associated interjacent spaces such as ribbon 46 and space 48 should be chosen to have some intermediate value lying between A and A In Fig. 4 where only three ribbons and three spaces are shown the combined width of ribbon 46 and space 48 should be equal to one-quarter of A where a is the main wavelength of the band of electromagnetic waves to be absorbed.

For example, an attenuator for a frequency band extending from 8 to kilomegacycles should have its first resistive ribbon equal in width to 3.75 millimeters and its last resistive ribbon equal in width to 4.7 millimeters. The intermediate ribbon progressively increases along the length of the support 14. The interjacent space is equal in width to the associated resistive ribbon.

It should be understood that the attenuator of Fig. 4, which has only three ribbons and three spaces, is shown by way of example only. It will be obvious to those skilled in the art that when greater absorption, as obtainable by the embodiment of Fig. 4, is desired the length of the attenuator element is increased and addi tional ribbons and spaces are added. In such a case,

the combined width of a ribbon and an interjacent space 4 should progressively increase from a value of one-quarter of A to one-quarter of A Fig. 6 depicts the results obtained with the different electromagnetic wave attenuators of this invention and makes a comparison with a prior art attenuator. The frequency in kilomegacycles per second are plotted on the abscissa 62 and the attenuation in decibels on the ordinate 64. Curve 66 represents the attenuation of a conventional IRC card, that is of a prior art attenuator having a continuous coating, inserted in a rectangular waveguide such as shown in Fig. 1. As can be seen, at 9.0 kilomegacycles, the attenuation was approximately equal to 30 decibels. Curve 68 shows the result obtained with the electromagnetic wave attenuator shown in Fig. 3 having the same dielectric support and the same coating material. As can be seen the attenuation was increased from 30 to 75 decibels. It may also be noticed that there was a slight decrease in the broadband characteristics of this attenuator. Curve 70 shows the results obtained with the electromagnetic wave attenuator shown in Fig. 4 where the width of the resistive strips and the width of the interjacent spaces was progressively increased from that of the smallest wavelength to that of the largest wavelength in the frequency spectrum. As can be seen from curve 70, the attenuation is still considerably larger than that obtained with a conventional card (curve 66) and the broad-band characteristics are vastly increased.

It is well known to those skilled in the art that the introduction of a solid body such as the attenuator 12 of Fig. 1 into a waveguide will create a disturbance in the main power path and produce reflections of the power incident thereon. In order to provide a good voltage standing-wave ratio, it has been found necessary to match the attenuator to the main power path. The matching techniques employed with conventional attenuators, such as quarter-wave transformer terminations or gradual tapered terminations, have been found equally well suited with the attenuator of this invention. In Figs. 5a to 5d, terminations S4, 56, 58 and 60 are shown by way of example and may provide suitable configuration of the end portions of the support 20.

Thus, there has been shown and described a novel low-power attenuator for providing improved absorption per unit length and which may be made less frequency sensitive than attenuators heretofore known. The attenuator of this invention may be utilized in all instances where dielectric bodies having a continuous resistive coating afiixed thereto have found application in the past. The attenuator of this invention has been described and shown in connection with decreasing the power transmitted from one end of the waveguide to the other. It will be obvious to those skilled in the art that the attenuator may likewise be utilized as a low-power load termination. Further, the attenuator of this invention may be movably mounted within a waveguide to provide variable attenuation in the same manner and with the same arrangement heretofore employed with the continuous coating type attenuator of the prior art.

What is claimed is:

1. An attenuator adapted to be disposed in waveguide means for attenuating electromagnetic energy propagated along said waveguide, said attenuator comprising a sheet of dielectric material adapted to be disposed in said wave- H guide parallel to the electric field and to the direction of propagation of said energy, a plurality of separate resistive strips disposed on said sheet, said strips extending transversely of said sheet and being disposed at one quarter of the wavelength of the energy to be attenuated, each of said strips being equal in width to approximately one-eighth of said wavelength whereby each of said strips will be spaced from the adjacent strips by one-eighth of a wavelength.

2. An attenuator adapted to be disposed in waveguide 5 means for attenuating electromagnetic energy propagated along said waveguide, said attenuator comprising a sheet of dielectric material adapted to be disposed in said waveguide so as to extend transversely of said waveguide parallel to the electric field of'said energy and longitudinally of said waveguide parallel to the'direction of propagation of said energy, a plurality of strips of resistive material extending transversely of said sheet, each of said strips having a width equal to substantially oneeighth of the wavelength of a frequency in said band I and being spaced from the adjacent strip by approximately one-eighth of the wavelength of said frequency.

UNITED STATES PATENTS 1 Bowen Nov. 21, 1950 Hewitt July 8, 1952 Bowen July 15, 1952 Weber Apr. 5, 1955 Dibos Apr. 10, 1956 Weber Sept. 23, 1958 Barnett Oct. 28, 1958 FOREIGN PATENTS Great Britain Nov. 22, 1940 

